Tunable micro-optic architecture for combining light beam outputs of dual capillary polarization-maintaining optical fibers

Abstract

A tunable PM fiber combiner is configured to be accurately alignable with and operative to combine into a single composite beam a pair of non-collimated, orthogonally polarized light beams transported over polarization maintaining (PM) optical fibers, whose mutual spatial separation may vary. The combiner includes birefringent elements, that are linearly or rotationally displaced to realize the composite beam. The resulting composite light beam may then be readily coupled (e.g. via a single mode fiber) to a downstream beam processing device, such as a Raman amplifier.

What is claimed:  
     
       1. An optical beam combining architecture for combining first and second non-collimated, mutually orthogonally polarized light beams, coupled thereto over polarization maintaining (PM) optical fibers whose mutual spatial separation may vary, into a single fiber for application to a downstream single beam processing device, said architecture comprising first and second birefringent walk-off crystal elements coupled in beam travel directions of said first and second mutually orthogonally polarized light beams and being displaceable relative to one another, so that the combined walk-off effect of said crystal elements is adjustable and compensates for variations in said mutual spatial separation of said PM optical fibers, and thereby brings said mutually orthogonally polarized light beams into coincidence for transport over said single fiber. 
     
     
       2. The optical beam combining architecture according to  claim 1 , wherein said PM optical fibers are respective fibers of a dual Panda-eyed capillary structure. 
     
     
       3. The optical beam combining architecture according to  claim 1 , wherein said first and second birefringent walk-off crystal elements respectively comprise first and second birefringent walk-off crystal wedges. 
     
     
       4. The optical beam combining architecture according to  claim 3 , wherein said first and second birefringent walk-off crystal wedges are linearly translatable relative to one another. 
     
     
       5. The optical beam combining architecture according to  claim 1 , wherein said first and second birefringent walk-off crystal elements are rotationally translatable relative to one another. 
     
     
       6. The optical beam combining architecture according to  claim 5 , wherein said first and second birefringent walk-off crystal elements comprise 45° crystal elements. 
     
     
       7. The optical beam combining architecture according to  claim 6 , wherein said PM optical fibers are respective fibers of a dual Panda-eyed capillary structure having mutually orthogonal Panda-eyes spatially oriented at +45° and −45° relative to a line passing through their respective fiber cores. 
     
     
       8. The optical beam combining architecture according to  claim 7 , wherein said PM optical fibers of said dual Panda-eyed capillary structure have a nominal fiber core separation D o , and wherein each of said 45° crystal elements has a thickness that provides a nominal walk-off of 1.414*D o  for a prescribed beam polarization orientation relative to respective walk-off axes between beam input and exit output faces thereof. 
     
     
       9. The optical beam combining architecture according to  claim 8 , wherein said 45° crystal elements are rotationally adjustable, so that, for a larger than nominal separation (D>D o ), said crystals are rotated to increase the angle between their walk-off axes and a line through said cores to values larger than +/−45°, and thereby realize coincident overlap of said mutually orthogonally polarized light beams at an increased distance walk-off axis intersection location, and for a smaller than nominal separation (D<D o ), said crystals are rotated to decrease the angle between their walk-off axes and said line through said cores to values smaller than +/−45°, so as to realize coincident overlap of said two mutually orthogonally polarized light beams at a relatively closer than nominal walk-off axis intersection location. 
     
     
       10. A method for combining first and second non-collimated, mutually orthogonally polarized light beams transported over polarization maintaining (PM) optical fibers, whose mutual spatial separation may vary, into a single fiber for application to a downstream single beam processing device, said method comprising the steps of: 
       (a) placing first and second birefringent walk-off crystal elements in beam travel directions of said first and second mutually orthogonally polarized light beams; and  
       (b) controllably displacing said first and second birefringent walk-off crystal elements relative to one another, so as to adjust the combined walk-off effect of said crystal elements and thereby compensate for variations in said mutual spatial separation of said PM optical fibers, and bring said mutually orthogonally polarized light beams into coincidence for transport over said single fiber.  
     
     
       11. The method according to  claim 10 , wherein said PM optical fibers are contained in a dual Panda-eyed capillary structure. 
     
     
       12. The method according to  claim 10 , wherein said first and second birefringent walk-off crystal elements respectively comprise first and second birefringent walk-off crystal wedges. 
     
     
       13. The method according to  claim 12 , wherein step (b) comprises linearly translating said first and second birefringent walk-off crystal wedges relative to one another. 
     
     
       14. The method according to  claim 10 , wherein step (b) comprises rotating said first and second birefringent walk-off crystal elements relative to one another. 
     
     
       15. The method according to  claim 14 , wherein said first and second birefringent walk-off crystal elements comprise 45° crystal elements. 
     
     
       16. The method according to  claim 15 , wherein said PM optical fibers are installed within a dual Panda-eyed capillary structure having mutually orthogonal Panda-eyes spatially oriented at +45° and −45° relative to a line passing through their respective fiber cores. 
     
     
       17. The method according to  claim 16 , wherein said PM optical fibers are respective fibers of a dual Panda-eyed capillary and wherein said PM optical fibers of said dual Panda-eyed capillary structure have a nominal fiber core separation D o , and wherein each of said 45° crystal elements has a thickness that provides a nominal walk-off of 1.414*D o  for a prescribed beam polarization orientation relative to respective walk-off axes between beam input and exit output faces thereof. 
     
     
       18. The method according to  claim 17 , wherein step (b) comprises, for a larger than nominal separation (D>D o ), rotating said crystals to increase the angle between their walk-off axes and a line through said cores to values larger than +/−45°, and thereby realize coincident overlap of said mutually orthogonally polarized light beams at an increased distance walk-off axis intersection location, and for a smaller than nominal separation (D<D o ), rotating said crystals to decrease the angle between their walk-off axes and said line through said cores to values smaller than +/−45°, so as to realize coincident overlap of said two mutually orthogonally polarized light beams at a relatively closer than nominal walk-off axis intersection location.